Adenosine triphosphate (ATP) is known to serve as a source of energy in cells and a substrate of phosphorylation, as well as an extracellular messenger. It is known that ATP is released from a cell by various stimulation such as cellular injury, inflammation, nociceptive stimulus, reduced blood oxygen level, and also known to be released together with another messenger from a primary sensory nerve terminal. (Non-Patent Document 1) ATP thus released mediates various extracellular signal transductions through an ATP receptor.
ATP receptor is categorized into ionotropic P2X family and G protein-coupled P2Y family. For P2X family, seven subtypes have been reported, and a member of this family forms a homo-trimeric structure or a hetero-trimeric structure together with another member of this subtype and functions as a non-specific cation channel.
The P2X7 receptor, a non-selective cation channel, belongs to the P2X family, and forms a homo-trimeric structure. Activation of P2X7 by extracellular ATP allows for the passage of cations across the plasma membrane. Prolonged or repeated ATP stimulation leads to the pore formation of pannexin hemichannel, and induces the cellular activation following the release of small molecule such as ATP. (Non-Patent Document 2) It is reported that the activation of P2X7 is involved in inflammation, immune and pain by the maturation and secretion of proinflammatory cytokines such as interleukin-1 beta and interleukin-18. (Non-Patent Document 3) Thus, it is known that the P2X7 receptor is involved in pain, central nervous system disease, immune disease and inflammatory disease. (Non-Patent Document 7-8, and Patent Document 1)
P2X7 is distributed in macrophages, mast cells, microglia, and astrocytes. It is known that disruption of the P2X7 receptor gene abolishes chronic inflammatory and neuropathic pain. (Non-Patent Document 4) It is reported that the P451L mutation of the mouse P2X7 gene has impaired pore formation and shows less mechanical sensitivity of neuropathic pain model mice. (Non-Patent Document 5) Additionally, an association between lower pain intensity in chronic pain patients and the hypofunctional allele of P2X7 has been reported, suggesting that P2X7 antagonist is useful in the treatment of chronic pain such as rheumatoid arthritis, osteoarthritis and neuropathic pain.
Additionally, it has been reported that P2X7 may be involved in multiple sclerosis, spinal cord injury, stroke, Alzheimer's disease, and depression (Non-Patent Document 6), suggesting that P2X7 antagonist is useful in the treatment of these central nervous system disease.
The compounds having an antagonistic activity for the P2X7 receptor are described in Patent Document 2. However, the compounds have different chemical structures from the compounds of the present invention.
The compounds having similar chemical structures to the compound of the invention are described in Patent Documents 3-6, Non-Patent Documents 7 and 8. However, there is neither disclosure nor suggestion about an antagonistic activity for the P2X7 receptor.